OK - is the lawn full of brown patches because dead leaves were left to lie on it from Autumn? Or because DC's play football all the time?
A lot of lawns don't look too good at this time of year especially if the ground is very wet but will improve.
So - rake up any dead leaves. Once ground is not squidgy underfoot, you could rake lawn with a lawn rake. This will get out any dead bits and moss etc and let the lawn breathe a bit. This is a backbreaking job though so think twice if the lawn is big! Small patches of mud and brown will probably sort themselves out within a month or so. With larger patches, you could buy some grass seed and follow instructions in packet to sow in bare patches. If your garden is constantly being used for football practice though, there is not much hope unless you can keep the DC's off the worst areas for a couple of months!
Get a compost bin and start composting raw fruit and vegetable waste, autumn leaves, coffee grounds and some cardboard ( to stop the mix from getting too slimy). This will save you loads of money in the long run. Meanwhile, pep up borders with some granular feed like growmore. Look at the texture of the soil. If it is very sticky and clay-like or extremely sandy then it would benefit from something like compost, but don't get peat-based if you want to be environmentally friendly (growbags are often the cheapest way to buy it). If you know any stables nearby, a better bet would be some well rotted horse manure. It doesn't smell once rotted. No need to dig anything in. Spread it on borders and the worms will do the rest.
When planting in borders, sprinkle some growmore or a mix of soil and manure into the planting hole first.
For borders, think about structure - do you want some evergreen plants to provide a constant backdrop? I like euonymus - you can get different variagated colours. If you have an independent garden centre you could go there and ask advice on what to plant. For low maintenance maybe think of a variety of evergreen plants of different heights and then intersperse with bulbs and flowering plants.
The biggest tip for instant improvement is "remove the brown" so get all dead leaves and twigs etc off beds. Then trim back anything that is looking untidy or straggly
Hope this helps!